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Acute Flaccid Myelitis


Illustration of the brain on a blue background

Acute flaccid myelitis (AFM) is also called “acute flaccid paralysis with anterior myelitis” or “polio-like syndrome.” It is rare and mainly found in children. It affects the body’s nervous system, specifically the spinal cord. AFM can be caused by some viruses, environmental toxins, and genetic disorders.

Viruses that can cause AFM include enteroviruses (polio and non-polio) and flaviviruses such as West Nile Virus, Japanese Encephalitis virus, or St. Louis encephalitis virus. Other viruses that may cause AFM are herpesviruses (such as cytomegalovirus and Epstein-Barr virus) and adenoviruses.

While AFM can cause your arms or legs to become weak, genetic disorders, environmental toxins, and Guillain-Barré syndrome can cause similar symptoms. Even with testing, sometimes the cause of AFM is never found.

AFM 101

AFM sometimes occurs after a viral illness that causes a fever and respiratory or gastrointestinal symptoms. These symptoms can include:

  • Arm or leg weakness that comes on suddenly
  • Loss in muscle strength and reflexes
  • Facial droop or weakness
  • Droopy eyelids
  • Trouble moving eyes
  • Trouble swallowing
  • Slurred speech
  • Loss of appetite
  • Respiratory failure (rarely)

Contact your doctor as soon as possible if you notice any of these symptoms in you or your child, for example if your child is not using their arm or leg normally.

AFM is often hard to diagnose. Providers look at the nervous system, location of muscle weakness, reflexes, and muscle firmness. Magnetic resonance imaging (MRI), tests of cerebrospinal fluid (CSF), and electromyography (EMG) nerve tests can be done to test for other illnesses or complications.

There is no specific treatment for AFM. The treatment depends on the individual.

In order to prevent AFM, you must first prevent the infections that can cause it. Take these basic steps to decrease your risk:

  • Wash hands frequently to remove germs
  • Cover coughs or sneezes
  • Stay home when you are sick
  • Make sure you and your family are up to date on vaccination
  • Take steps to prevent mosquito bites

  • AFM Fact Sheet, P-01298 (Multiple Languages): Educational fact sheet for the general public on AFM covering signs and symptoms, treatment, and prevention
  • About AFM: CDC (Centers for Disease Control and Prevention) webpage, including information on symptoms, diagnosis, possible causes of AFM, treatment, and prevention
  • AFM Resources: CDC webpage containing resources for patients and their families

Provider Information

Please report all cases with onset of acute flaccid limb weakness to the Division of Public Health within 24 hours (after hours reporting is not required) by calling the Bureau of Communicable Diseases at 608-267-9003.

Clinical Criteria

An illness with onset of acute flaccid limb weakness (AFM case definition)

Laboratory Criteria
  • Confirmatory Laboratory Evidence: a magnetic resonance image (MRI) showing spinal cord lesion largely restricted to gray matter* and spanning one or more vertebral segments
  • Supportive Laboratory Evidence: cerebrospinal fluid (CSF) with pleocytosis (white blood cell count >5 cells/mm3)
Case Classification


  • Clinically compatible case AND
  • Confirmatory laboratory evidence: MRI showing spinal cord lesion largely restricted to gray matter* and spanning one or more spinal segments


  • Clinically compatible case AND
  • Supportive laboratory evidence: CSF showing pleocytosis (white blood cell count >5 cells/mm3)

* Spinal cord lesions may not be present on initial MRI; a negative or normal MRI performed within the first 72 hours after onset of limb weakness does not rule out AFM. MRI studies performed 72 hours or more after onset should also be reviewed if available.
Terms in the spinal cord MRI report such as “affecting mostly gray matter,” “affecting the anterior horn or anterior horn cells,” “affecting the central cord,” “anterior myelitis,” or “poliomyelitis” would all be consistent with this terminology.


To provide consistency in case classification, review of case information and assignment of final case classification for all suspected AFM cases will be done by experts in national AFM surveillance. This is similar to the review required for final classification of paralytic polio cases.

The CDC also requests that clinicians collect and submit the following specimens to the CDC for testing as early as possible in the course of the illness:

  • Cerebrospinal fluid
  • Blood
  • Stool

Submission of nasopharyngeal or nasal (mid-turbinate) plus oropharyngeal swab specimens is requested only if the patient tests positive for enterovirus or rhinovirus at an external lab.

Pathogen-specific testing should be performed locally at hospital or state public health laboratories. Specimens submitted to the CDC are not intended for clinical diagnosis

Last revised March 14, 2023